How do motherboards degrade? Does the solder on the motherboard affect this process? Or the aging of the chipset?
The soldering work should not cause issues unless it's a manufacturing fault. Given that solder usually withstands 200-300°C heat would also result in other failures way beforehand.
In most cases it's one/multiple of the Voltage Regulation Modules (or actually circuits) that give up, mostly duo to capacitor failure.
If you look at a motherboard around the CPU socket there are a bunch of capacitors, MOSFETs and inductors. Groups of them form VRMs for the various parts on a motherboard e.g. the CPU itself (Vcore, VTT).
Being located right next to the CPU means that they are exposed to more heat than just room or case temperature. If your CPU cooling is not efficient enough they can certainly run quite hot. So over time those components degrade, mostly the capacitors.
An unfortunate combination of heat, humidity and load can further increase the degradation.
Next in line but for the same reasons would be the VRMs next to the add-on sockets, mostly the PCI-Express slot holding your GPU.
Not sure it counts as degradation but the physical abuse, bumps, are probably next in line. If you look at all those non-boxed coolers they are all quite massive and only held in place by a few screws and usually a back plate. What you consider just a little bump against the case puts a lot of strain on the socket area of the motherboard if one of those big coolers is installed.
I guess too many hard bumps over time could actually cause some soldering to break lose.
I've heard that after a certain heat/cooling cycle, the solder could produce a "crack" and results in an unbootable machine (caps lock blinking & black screen) since the the connection is messed up.
Unless you have a laptop, the caps lock LED is controlled by the circuit of your keyboard, not by your motherboard. So there is no directly correlation between it blinking and the machine not booting.
This is just an observation e.g. when the motherboard keeps power cycling for whatever myriad of reasons – the keyboard receives power, lights up, loses power, LEDs go off.
This could also happen if your computer is in perfect condition but the circuitry of the keyboard is broken.
Equally an "unbootable machine" does not mean that something is physically or technically broken or cracked. It's just an observation with a multitude of possible causes.
Really, the kind of heat-cooling cycle required to damage solder on a motherboard or other components is unlikely to happen outside a laboratory and as previously stated, other components will fail duo to that kind of temperatures long before that.
All components on a motherboard _can_ degrade, but it is very uncommon. It's not unusual to see PCs 15 or 20 years old and still working, and in contrast, it's very rare to see an old motherboard die all of a suddden. Still, here are some failures I've come across:
1. CMOS battery. Not stricly "motherboard" but it's on the motherboard. These need replaced every 5 years or so.
2. Capacitors. This is not a common problem, but some batches of capacitors (particularly those made by certain Japanese companies around the year 2000, can't remember the details) tend to leak and eventually "blow".
3. If a component runs hot, its life will be shortened. This tends to affect particularly CPUs that have been overclocked, but on systems that have poor ventilation or run hot for other reasons it can affect other components too.
4. Hairline cracks in the motherboard. Once again, this is not common at all, and it's not really a function of age but mostly of mishandling, but it does happen and can cause the motherboard to fail completely or partially.
5. I've never heard of, let alone seen, a motherboard which was corroded. It's possible in theory, perhaps in warm tropical weather by the sea, or perhaps due to fungal infection, that a motherboard would fail in this way, but it is not "normal degradation" by any means.